Don't play the Chinese lines (pretty mediocre), Polish line (pretty bad grind for newer players), or Czech line (many apparently are a big pile of poop).

It depends on what you want. If you want to be able to take hits reliably, Russian heavies. Japanese heavies may seem OP with armour, but their mobility and size let them down when matchmade against higher tiers. Russian heavies at least usually have mobility or super trolly turret armour that will still be able to help you when you're facing higher tiers.

If you want to have an all-round decent package that can snipe, flank, and do almost anything, Russian mediums. Pretty good guns, mobility, and sometimes trollish armour on a medium chassis.

If you want to see one shots and have laughs, M4 Sherman or KV-2. Two best mid-tier derp guns. Pz. 4H is also an option.

If you want powerful guns that can pen a lot of things mounted to a reasonably mobile chassis, other than Russian mediums, French heavies are an option. They're difficult to play, but once you put the gun to work, it does its job amazingly. Pretty punchy, can fight higher tiers effectively after Tier 5, mobile, not much armour.

All depends what sounds good to you. Russian heavies and mediums are a good starting point. The other ones are if you want to have fun in other ways.